1. Technical Field of the Invention
This invention relates generally to energy monitoring systems and more specifically to electrical power monitors as used in residential energy management systems.
2. Background Art
There is a broad growing interest in energy conservation. Reducing residential electrical consumption is a priority for national, state, and local governments, for utilities, and for consumers. It is especially important to reduce electrical consumption during peak load periods, typically in the afternoon on hot days. It is generally accepted that consumers are more likely to change their behavior if they are made aware of where, when, and how they use electricity in their household and how much it costs. A power monitoring system provides this information.
The simplest monitor displays the present rate of power use. Its function is similar to the power meter used by the utility company for billing, but it displays the information in a format and location that this more convenient for consumers. The power can be displayed in units of electricity (kilowatts) or in units of cost ($x.xx per hour). The display can be portable or located in a convenient place in the home. This type of monitor can make the consumer aware of the total cost of the electricity used by the household at the present time, and the power can be accumulated to show the total power used during a period of time and its cost. U.S. Pat. No. 7,174,260 issued Feb. 6, 2007 to Tuff, et al. describes such power monitor. It uses an optical sensor to monitor the power meter wheel as it turns and processes this information into rate of power use and cost. This information is wirelessly sent to a display device located in the residence. The device also accumulates the total power used during a period of time and its cost.
Other monitors extract information about the status of an electrical device by analyzing the amount of power used as a function of time. A power measuring device is used for each monitored electrical device and the power measurements are communicated to a central processor. Analysis of the changes in power use can determine the status of the device which is then displayed. U.S. Pat. No. 6,906,617 issued Jun. 14, 2005 to Van der Meulen describes such a system. The disadvantage of this approach is that a separate complex power monitor is used for each electrical device.
More sophisticate monitoring systems record the energy used over a period of time and processes and/or displays the information to make it more useful to the consumer. For example a graph of power use versus time can show the consumer how power use varies with household activity. As another example, the consumer might specify a start time and an end time, and the monitor might respond with the cost of power used during this period. U.S. Pat. No. 7,246,014 issued Jul. 17, 2007 to Forth, et al. describes such a system and describes several ways to display the information.
An increasing number of utilities are charging different rates for power depending on the time of day, day of week, or based on the total electricity used during the billing period. Some monitoring systems integrate this information with the measured power to provide the consumer more accurate and useful information that might encourage them to reduce energy consumption and/or change when they use power. For example, they might chose to reduce the amount of air conditioning used when electricity is expensive and to run their pool pump or do laundry at times when electricity is less expensive. U.S. Pat. No. 5,289,362 issued Feb. 22, 1994 to Liebl, et al. describes a system where a variation in the cost of electricity alters the settings for HVAC equipment.
While knowing the power and total power used by a household has value, there is more value in knowing the power and total power used by individual electrical devices. This information enables the consumer to make informed decisions about altering usage patterns and/or upgrading to new and/or higher efficiency devices. There exist power monitors that can be connected to an individual power circuits, individual outlets, and to individual devices. The disadvantage of this approach is an additional power sensor and a method of reporting its measurements are needed for each device monitored.
U.S. Pat. No. 4,858,141 issued Aug. 15, 1989 to Hart, et al. discloses a non-intrusive appliance monitor apparatus. It discloses a technique to track the power used by individual electrical devices based on analysis of the variations in total power used by a collection of devices. It is based on the observation that different appliances use different amounts of power, so a step change in total power can be attributed to a device in the group that uses power at the rate corresponding to the measured step change. The times when the step increases and decreases in power occur are recorded and associated with a specific device to calculate the total power used by that device during a predetermined period of time. The disclosure also uses step changes in the measured reactive power as further discriminator between appliances that use nearly the same real power. The technique disclosed can reliably track a small number of appliances with specific electrical properties and that use different amounts of power.
However, Hart et al. has many limitations that make it impractical for use in most modern residential homes. Modern homes have dozens of different devices. Many of the devices of interest use hundreds of watts, so these devices generate similar turn on and turn off steps that can not be separately tracked. Many devices use variable power while they are on, so the turn on and turn off steps do not match; therefore they can not be accurately tracked. Many electronic devices use power in short frequent pulses. These frequent pulses will often occur at nearly the same time as the turn on steps and turn off steps of other appliances. This introduces large uncertainty in step measurements, especially at the measurement rate of one sample per second preferred by Hart et al. The disclosure states:
“The power consumed by the residence is preferably measured approximately every second. The exact rate is not critical to the method. A slower rate can be used, but this leads to more frequent errors by the device. A slightly higher rate may be preferable, but there is no apparent advantage in exceeding approximately ten measurements per second. The measurements need not be at regular intervals.”
Hart et al. teaches the method of using admittance scaling to eliminate the effects of voltage variation on the measured power. Modern devices with electronic control often behave differently. For example, some devices use a constant amount of power independent of the supply voltage.
Some electrical devices turn on and off very slowly. For example HVAC fans may take a minute or more to reach full speed, so the rate of change is very small. Such a step is difficult to detect given the disclosed behavior of the “steady state detector 22”.
Hart et al. discloses a technique that works only for very limited circumstances, and does not provide a practical method of tracking multiple electrical devices in a typical residential home.
U.S. Pat. No. 5,483,153 issued Jan. 9, 1996 to Leeb, et al. discloses a transient event detector for use in non-intrusive load monitoring systems. It discloses a technique to identify different devices by analyzing variations of electrical parameters during turn on transitions. These turn-on transient variations are captured as envelops of harmonic content of parameters such as real power, reactive power, and current. These envelops are different for different classes of electrical devices. Sections of the envelop that vary rapidly are called “v-sections”, and only these portions of envelops are used in the identification of devices. It specifically states:
“Searching for complete transients is an undesirable approach because it limits the tolerable rate of event generation. No two transient events could overlap significantly if each transient were to be identified correctly. Instead, the transient event detector in accordance with the present invention searches for a time pattern of segments with significant variation, or “v-sections”, rather than searching for a transient shape in its entirety.”
A further limitation is illustrated in FIG. 7 with the description: “The overlap condition in FIG. 7 would not generally be tractable, since the instant start v-section and the first induction motor v-section overlap severely.” A v-section “locked out” behavior is disclosed based on the “assumption that v-sections should not overlap”.
Leeb et al. discloses an improved technique of transition analysis compared to Hart et al. It is able to associate turn-on transients more accurately and to a broader range of different electrical devices. However, Leeb et al. does not disclose how to make a practical power monitor for residential home because it provides only modest improvement in the ability to deal with overlapping turn on transients and provides no improvement in associating turn off transients with devices that are on. Therefore it does not enable tracking individual electrical devices in a typical residential home.
What is missing in the prior art for power monitors is a practical and economical method of using a single central power monitor to provide detailed information about the power and total power used by multiple individual electrical devices.